The invention generally relates to a fault-current protective device, which operates with a tripping (triggering) circuit. Preferably, it includes the functions of xe2x80x9cdetection of a fault currentxe2x80x9d and xe2x80x9ctripping in order to disconnect a network that is to be protectedxe2x80x9d.
Devices for protection against fault currents can be designed on the basis of various types of functional principles: they may be designed to be independent of the network voltage, and are then generally referred to as fault-circuit breakers. Alternatively, they may require energy from the network for tripping, when they are dependent on the network voltage. These are generally referred to as differential-current circuit breakers. The expression fault-current protective device is in this case intended to cover both types of devices for protection against fault currents, jointly.
The design of known general fault-current protective devices is generally known, for example, from xe2x80x9cetzxe2x80x9d (1986), Issue 20, pages 938 to 945. FIGS. 1 to 3 there show outline circuit diagrams and functional principles of an FI circuit breaker which is independent of the network voltage, and of a DI circuit breaker which is dependent on the network voltage.
Fault-current protective devices are intended to ensure protection against direct contact with a live conductor and to assure such protection against indirect contact, protection against touching a conductive component on which a voltage with respect to ground has built up owing to a fault.
Fault-current protective devices of both types generally operate using a core balance transformer, which is used to form the residue of the currents in conductors which are passed through it and are to be protected. In the event of a fault current, a voltage signal is induced in a secondary winding, and this voltage signal is evaluated. Thereafter, via a tripping relay and via a switching mechanism, it leads to the opening of switching contacts in the power supply to be protected. The tripping relay of a fault-current protective device is in this case inductively coupled, via the core balance transformer, to the network to be monitored. In the case of a differential-current protective device, network energy is also conductively coupled into a circuit having an amplifying effect.
The tripping response of fault-current protective devices is dependent, inter alia, on the frequency of the fault current (DE-A-196 34 438). The relationships in this case are known, as to where protection is provided against ventricular fibrillation and with regard to the areas in which it is still possible to let go of a live conductor if it is touched directly.
An embodiment of the invention is generally based on an object of developing a fault-current protective device which ensures protection against indirect contact as well as fire protection.
Such an object can be achieved by a fault-current protective device. The tripping circuit can be matched to keep the tripping fault current below a tripping limit curve (B) for fire protection, especially for fault current frequencies, in the tripping circuit, beyond a stricter limit curve (A) for protecting personnel against indirect contact. An embodiment of the invention is based on a combination of the requirements for personnel protection and for fire protection relating to a suitable fault-current protective device. This interaction is preferably governed by the relationship:                               I          limit                ⁡                  (          f          )                            I                  Δ          ⁢                      xe2x80x83                    ⁢          n                      =                  F        ⁡                  (          f          )                    =              B        =                  const          .                      ,
where                     I        limit            ⁡              (        f        )              =                  W        fire                    U        network              ,
where
Ilimit(f) is the maximum fault current limit, at which there is still no risk of fire, for the respective frequency,
Ixcex94n is the rated tripping fault current at the network frequency, in order to take account of personnel protection,
Wfire is the power limit, in watts, required for fire protection,
Unetwork is the network voltage of the network to be protected, and
F(f) Is a so-called frequency factor.
In this case, the tripping characteristic of fault-current protective devices can be defined for indirect contact and as function of frequency.
Such a fault-current protective device takes account of the increasing number of appliances which produce fault currents at a relative high frequency in the event of a fault. Such appliances can include, for example frequency changers and appliances with pulsed power supplies.
With regard to indirect contact, personnel protection can be based on the idea of defining a rated fault current for a maximum permissible ground resistance at a network frequency. This, thus ensures a maximum permissible contact voltage with respect to ground, in a manner known per se, for protection against indirect contact in the event of a fault. Thus, for example, the maximum permissible contact voltage with respect to ground at a network frequency of 50 to 60 Hz is normally defined to be 50 V. Accordingly, a maximum ground resistance Rground in accordance with the relationship             R      ground        =                  U                  m          ⁢                      xe2x80x83                    ⁢          ax                            I                  Δ          ⁢                      xe2x80x83                    ⁢          n                      ,
where
Umax is the maximum voltage involved which is not going to be dangerous for indirect contact, and
Ixcex94n is once again the rated tripping fault current at the network frequency, in accordance with limit curve A, for protecting personnel against indirect contact.
Other known requirements for personnel protection can also be taken into account in the limit curve A, instead of this relationship.